Help Me Understand The (NEW TAKE OF THE) Single Photon Double-Slit Experiment

Several weeks ago I picked up “The Elegant Universe” because I thought it would have information on The Ultimate Theory. Soon afterwards, I picked up “How to Teach Physics To Your Dog” in order to get a better handle on some of the concepts in the beginning of “The Elegant Universe”

Things with HTTPTYD were going well, until I got to the part about how the double-slit experiment…when performed with an apparatus that can fire single photons at a time through the double-slit…demonstrated how a quantum particle theoretically goes through both slits at the same time. The book (and the Wikipedia article I went to for help) explains that the resulting interference pattern that develops over time, during this experiment, somehow demonstrates this.

To me, that the interference pattern emerges after a period of time spent firing single photons at a double-slit, shows the unmeasurable nature of quantum particles that was explained in the book earlier. If you fire a single photon at the double slits, the result is a single spot on the other side in a random location, which makes sense…not two spots on the other side in random locations, which I would expect if the photon is going through both slits at the same time.

What am I missing?

EDIT: If it helps, while I'm not a scientist, or a physicist, I understand the Heisenberg Uncertainty Principle...not the equation but the concept behind it...and I understand the Schrodinger's Cat thought experiment. I'm pretty sure I understand the quantum eraser experiment, and I know that wave-particle duality is the concept that particles can behave like a wave and a particle.

The pattern behind the double slit is the result of interference between photons emerging from each slit, regardless of whether there's a lot, or if there's *only* one. Each individual photon leaves one spot behind. But if you keep firing one at a time and keep the previous spots, the interference pattern builds up.

I believe Feynman said that all of Quantm physics can be understood through the double-slit experiment. So far as far as I know, no-one has really understood it.

Ie. There are a bunch of different theories which try to explain the results...AFAIK none of which have reached a concensus yet.

My take, is that the photon doesn't really "exist"* until it hits the wall. Until then it's a wavefunction of a photon not a classical photon. Once it hits the wall the probabilities which previously existed in a wavefunction all coelsce (reduce) down into a single possibility. And thus we get a spot on the wall which we now call a photon.

If X is a wall/blocker with a single slit, nothing happens, it's (essentially) a straight line shot.

But photons of the same frequency will "interfere" with each other. Essentially causing each other to not go in a straight light.

Now, if X is a double split, the single photon will not go in a straight line.

In fact, it will look like it's been interfered with by a photon of the same frequency.

One of the theories is that, somehow, the single photon goes through both slits and then, somehow, interferes with itself.

How? There are plenty of theories, but not any single one that everyone can agree on. Maybe it's multiple universes, maybe a quantum wave is an accurate description and a split quantum wave can interfere with itself and then somehow communicate to the other half instantaneously when it collapses and have the other half collapse into non-existence. Maybe it's aliens messing with us, I don't know. But if I did there would be a nobel prize with my name on it sometime in the future.

Go here and watch video 6 on probability and uncertainty, and get the answer from the Feynman himself. Feynman's lectures are incomparable. One thing I really like about it is that he introduces the lecture by saying you should not try to understand why nature is this way, just accept that it is.

Particles do not have a particular location and velocity, they merely have probabilities of location and velocity. This does not mean we're making educated guesses about position and velocity, it means a particle is all the possible futures it can have. All but one of these futures collapses when it is observed. Meanwhile, these futures of the same particle can interfere with each other. Quantum probability is not a mere description of where a particle could be found and could be going, a mere mathematical abstraction, it is an actual property. The photon in the experiment is all the possible paths it can take, some through one slit, some through the other, and it is interfering with itself. For each photon, measuring devices record one possible outcome of this self-interference.

Yep. It's all complete, utter **** with highly accurate and verified models, is the basis of microelectronics and has transformed understanding of chemistry.

not two spots on the other side in random locations, which I would expect if the photon is going through both slits at the same time.

Why would you expect that? It is still only one photon. It can only be absorbed once. We say it goes through both slits because the results can't be described in terms of a particle going through one slit half the time and the other slit half the time. Beyond that, it really gets into semantics because the fundamental problem is that 'particle' and 'wave' are terms that try to phrase the quantum world in terms of classical objects. That description can only go so far, in the end quantum mechanics is the correct, fundamental description of the microscopic world, and classical analogies to waves or particles are only approximations that are sometimes useful.

Particles do not have a particular location and velocity, they merely have probabilities of location and velocity. This does not mean we're making educated guesses about position and velocity, it means a particle is all the possible futures it can have. All but one of these futures collapses when it is observed. Meanwhile, these futures of the same particle can interfere with each other. Quantum probability is not a mere description of where a particle could be found and could be going, a mere mathematical abstraction, it is an actual property. The photon in the experiment is all the possible paths it can take, some through one slit, some through the other, and it is interfering with itself. For each photon, measuring devices record one possible outcome of this self-interference.

So...if I'm understanding correctly...you cannot speak of "aiming" a photon anywhere, correct? So the reasoning that a single photon repeatedly fired through a single slit does not create an interference pattern because it's a "straight shot" is incorrect? Also, doesn't a single photon fired repeatedly through a single slit create an interference pattern...or is Wikipedia wrong yet again?

More importantly...and going back to my original question...I'm not sure I understand how what you posted explains my misunderstanding. Here's the part in the Wikipedia article that I'm not getting...which is almost identical the part in "How To Teach Physics To Your Dog" that I don't get

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Sending particles through a double-slit apparatus one at a time results in single particles appearing on the screen, as expected. Remarkably, however, an interference pattern emerges when these particles are allowed to build up one by one (see the image to the right).

Basically my question is: Why is this remarkable? The way I'm seeing it, the only difference between the interference pattern created from illuminating a double-slit and the interference pattern created from firing one photon at a time at double-slits is time. The interference pattern for the single photon double slit experiment takes more time than the other.

I don't see how slowing down the creation of the interference pattern says anything about one photon going through two slits at the same time. For that matter, I don't see how the illumination of double-slits says anything about one photon going through two slits at the same time.

warmachine wrote:

Yep. It's all complete, utter **** with highly accurate and verified models, is the basis of microelectronics and has transformed understanding of chemistry.

It is worth mentioning that if the slits are too wide and/or too far apart in comparison with the wavelength of the light you end up with two spots and apparent behaviour as if the photon was going through just one at a time. You should buy a laser pointer (it shoots out photons that are nearly identical to eachother) and shine it through a piece of foil with two holes, and observe it yourself.

Regarding the double-slit experiment with sufficiently narrow slits sufficiently close to each other:Under the Copenhagen interpretation, the photon goes through two slits, impacts the screen in the entire pattern, but is observed in just one spot, chosen according to the value of probability function, i.e. the spots where the interference is completely destructive are never chosen, which is an interesting case because it means that even a single photon, that was fired only ever once, can sense the both slits (if one is closed there will be no destructive interference). The observation is said to 'collapse' the wave function; until the observation is made, the photon has no definite position on the screen.

I really prefer the MWI interpretation of quantum mechanics. The photon goes through two slits, and impacts the screen in the entire pattern at once. The screen is thermally coupled to the environment and the environment becomes a zillion different non interacting universes in each of which the photon apparently hit the screen in just one spot. But that's rather advanced topic.

It is both correct correct and incorrect to write of aiming a photon because it can behave like a particle and like a wave. Particles can be aimed, waves aren't really aimed. Photons fired at a single slit effectively creates a single wave that doesn't interfere with itself because there's no other wave to interfere with. Double slits create two waves that can interfere with each other.

As for "Remarkably, however, an interference pattern emerges when these particles are allowed to build up one by one...", the remarkable thing is photons behave like particles yet can also behave like waves. Particles are self-contained objects, waves are oscillations in a widespread, compressible medium. These are mutually exclusive. Light being both wave and particle is nonsense yet it is.

Finally, my flippant remark describes the nature of science: it's all wrong. And in the case of quantum mechanics, it's also ridiculous. Even saying objects fall to Earth is just guesswork based on observation. It's just an amazingly accurate and immensely useful form of wrong. So accurate and useful, other methodologies for explaining the natural world are worthless in comparison. But it's still unproven guesswork.

Basically my question is: Why is this remarkable? The way I'm seeing it, the only difference between the interference pattern created from illuminating a double-slit and the interference pattern created from firing one photon at a time at double-slits is time. The interference pattern for the single photon double slit experiment takes more time than the other.

This is absolutely correct. There is no difference between the bright laser beam and 1 photon at a time. Both show exactly the same interference pattern. It was hard for people to accept that particles could do this. The important thing is that if you block first one slit and then the other and add up the intensity patterns, you don't get the interference. That means that you can't describe the photon as 'sometimes it goes left, sometimes right'. This bothered a lot of people. Some suggested that perhaps it was an interaction between multiple photons that gives rise to the pattern. The idea is that if you block one slit or the other, you stop the interaction. By sending only a single photon at a time, you can see that this is not the case.

Basically my question is: Why is this remarkable? The way I'm seeing it, the only difference between the interference pattern created from illuminating a double-slit and the interference pattern created from firing one photon at a time at double-slits is time. The interference pattern for the single photon double slit experiment takes more time than the other.

This is absolutely correct. There is no difference between the bright laser beam and 1 photon at a time. Both show exactly the same interference pattern. It was hard for people to accept that particles could do this. The important thing is that if you block first one slit and then the other and add up the intensity patterns, you don't get the interference. That means that you can't describe the photon as 'sometimes it goes left, sometimes right'. This bothered a lot of people. Some suggested that perhaps it was an interaction between multiple photons that gives rise to the pattern. The idea is that if you block one slit or the other, you stop the interaction. By sending only a single photon at a time, you can see that this is not the case.

Okay...this (the bolded parts)...helps me begin to understand what the big deal is. I just have one question from what you posted:

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The important thing is that if you block first one slit and then the other and add up the intensity patterns, you don't get the interference. That means that you can't describe the photon as 'sometimes it goes left, sometimes right'.

I'm missing something here...why would you expect to get interference when covering one slit, then the other, then combining the two?

Actually...back up. Don't you get an interference pattern when illuminating a single slit, provided it's narrow enough?

Actually...back up. Don't you get an interference pattern when illuminating a single slit, provided it's narrow enough?

No. You've forgotten the light bulb underneath a glass tank full of water and vibrating motor experiments in physics class. If there's a single slit, that's only one wave. There is no other wave to interfere with it.

Now here's the really weird bit. If you do the double slit experiment and fire single photons one at a time, and you TRACK which slit they go through, you do NOT get the interference pattern.

You only get the interference pattern when the information about the path taken is irretrievable.

The book used the quantum eraser to make this point…I got it up until the part in my original post. Doesn't the lack of an interference pattern happen because of the measurement that happens when you track the particle? And that is because measuring the position of the particle changes its momentum?

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No. You've forgotten the light bulb underneath a glass tank full of water and vibrating motor experiments in physics class. If there's a single slit, that's only one wave. There is no other wave to interfere with it.

I didnt take physics in high school and it looks like Wikipedia is wrong again…

Now here's the really weird bit. If you do the double slit experiment and fire single photons one at a time, and you TRACK which slit they go through, you do NOT get the interference pattern.

You only get the interference pattern when the information about the path taken is irretrievable.

The book used the quantum eraser to make this point…I got it up until the part in my original post. Doesn't the lack of an interference pattern happen because of the measurement that happens when you track the particle? And that is because measuring the position of the particle changes its momentum?

The 'standard' explanation as I understand it, is that tagging the photon with some kind of measurement collapses the probability wavefunction. Interestingly enough, if you tag the photon at one spot, but destroy that data, you get the interference pattern again.

The important thing is that if you block first one slit and then the other and add up the intensity patterns, you don't get the interference. That means that you can't describe the photon as 'sometimes it goes left, sometimes right'.

Why not? In the 'classical' particle picture, the two slit experiment would result in just two spots (actually a diffraction pattern for each slit). What the wavefunction picture tells us is that a photon can, statistically, appear in more than one location in space, with the shape of the wavefunction representing the statistical probability of finding a photon at a given location*. This can be reconciled with the particle picture by saying that no source creates a stream of photons (or electrons, etc) with exactly the same angular direction, i.e. different photons in the stream take slightly different paths.

So in the double-slit experiment, a bright spot appears due to photons from both slit sources hitting the same spot, resulting in enough photons to be detectable/seen. In the dark regions, the sum of photons from both slots is still too few to be detected. Dark spots don't mean that there are zero photons in that regions. Just that there is a reduced probability of finding a photon in the dark regions (in comparison to the probability of finding the photon in the bright regions). With a sensitive enough detetctor, the bands will still be there, but the 'dark' bands will just be less bright than the 'bright' bands.

* I'll note though that on the front page, a recent paper was covered which suggests that the wavefunction is not just a statistical picture, but somehow 'real'. I don't really understand what that means.

I hate sticking my nose into threads like this (though I love reading them) because I’m an idiot, but on the off chance that it might help someone else...

While wandering through Wikipedia articles and furrowing my brow in a vain attempt to understand anything, I came across the following paragraph from the entry on the Observer Effect in relation to quantum mechanics which seemed to make sense:

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This is demonstrated in a common thought experiment using the double slit setup. Imagine a double slit experiment where quantum particles are fired towards the two slits. The quantum particles pass through the slits and hit a momentum sensor a distance of D behind the slits. The momentum sensor has the ability to be turned off and on via a pin which stops the movement of the sensor when it is hit by a quantum particle. When the pin is in place, no measurement of the momentum can take place. When the pin is removed, the sensor can recoil when struck by a quantum particle and by measuring the recoil determine from which slit the quantum particle came. If the pin is removed and we can detect from which slit the particle came, then the wave-like passage through both slits cannot occur and no interference pattern will develop. However if we put the pin in place, and can no longer determine from which slit the particle passes through, then an interference pattern can develop.[3] This can be taken a step further using the delayed choice experiment.

...sense in that I vaguely understand the principle, but am still baffled as to why things should work that way. I realise the above example is also probably not very correct in some physical way, but is more just a mechanism to help picture the concept.

Actually...back up. Don't you get an interference pattern when illuminating a single slit, provided it's narrow enough?

No. You've forgotten the light bulb underneath a glass tank full of water and vibrating motor experiments in physics class. If there's a single slit, that's only one wave. There is no other wave to interfere with it.

There is an interference pattern from a single slit if the slit is wider than the wavelength, but it is usually referred to as diffraction. The diffraction pattern from a single slit is very different from the interference pattern from a double slit being discussed here, though. The single slit diffraction pattern is based upon the width of the slit, while the double slit interference pattern is based upon the spacing between the slits. You can have both effects occurring at the same time. See the first figure on the right in the wiki article on the double-slit experiment for an example.

My (possibly eroneous) attempt at an explanation for the double slit experiment. Using collaquial expressions.

You aim an object at a screen with two slits in it.

It goes through both slits at the same time.

It goes through both of the damn slits at the same time. It's like you going out the front door and the back door of your house at exactly the same time.

This is not possible.

But isn't it...with waves? Because they diffract around things?Yes, light is a wave AND a particle, but it's easy to imagine a stream of particles going through two slits at once. It's like pouring dog kibble into two bowls at the same time. Right?

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But it does.

A single object somehow manages to be in two places at the same time. This is stupid, this can't be real. But every single time someone does the experiment it keeps happening.

The universe is obviously broken.

That's a little harder to grasp...but I'm working on it

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And you know what's worse?

The object you send through the slits? It doesn't have to be a photon... they can do it with molecules. Your hand? It's made up on molecules. Your brain? Molecules.

The universe is obviously broken.

Er...but your hand is made up of a LOT of molecules. Again, I'm not a scientist but I am assuming that prevents you from being in two places at once. If it doesn't, or shouldn't, I want to know as soon as possible so I can start working on doing so as quickly as possible.

The important thing is that if you block first one slit and then the other and add up the intensity patterns, you don't get the interference. That means that you can't describe the photon as 'sometimes it goes left, sometimes right'.

Why not? In the 'classical' particle picture, the two slit experiment would result in just two spots (actually a diffraction pattern for each slit).

No, that is not true. By necessity, the two spots must overlap enough to be only a single spot, and the two-slit interference can only occur in the region that is illuminated by both slits.

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Dark spots don't mean that there are zero photons in that regions. Just that there is a reduced probability of finding a photon in the dark regions (in comparison to the probability of finding the photon in the bright regions). With a sensitive enough detetctor, the bands will still be there, but the 'dark' bands will just be less bright than the 'bright' bands.

Of course there is always some leakage due to imperfections, but it is nothing fundamental. If you design your equipment properly you can make the 'dark' spots arbitrarily dark. In a two-beam interferometer you can get the count rates on the dark port extremely low. 100:1 contrast is easy with ordinary equipment and care, and much better than that if you really work at it.

Ok sorry on iPhone so quoting is tricksome but as per dog kibble analogy. In this case there is only one piece of dog kibble. But it still goes into both bowls.

And if it was just a Particle travelling in a wave then we shouldn't get an interfernce pattern. The particle travelling in a wave pattern should just pass through one slit. Then make a wave out of that slit. No interference, instead we get a pattern whIch shows twos wave passing through each slit but we only sent one particle.

Disclaimer: I'm still in newbie stage on this stuff but this is what I've kenned on the matter so far.

One thing I really like about it is that he introduces the lecture by saying you should not try to understand why nature is this way, just accept that it is.

Oh, nonsense. Obviously I have the utmost respect for Feynman and his works, but the above is just wrong. If we did that we'd still be at "rain is caused by warring sky-spirits".

Much better advice is "first accept that nature is the way it is, then try to understand it."

troymclure wrote:

A single object somehow manages to be in two places at the same time. This is stupid, this can't be real. But every single time someone does the experiment it keeps happening.

This is a trap I've also fallen in myself, in the past. Your intuition is getting in your way. When you say 'single object' you automatically conjure an image of a tiny billiard ball in your head, and billiard balls can't be in two places at once. Stop it. The universe is not made of tiny billiard balls bouncing around. The universe has never been composed of tiny billiard balls bouncing around. The universe is running smoothly, as it always has. It's you that's confused, not the universe. (This is not a put-down. This is to say, thanks to modern science we have the opportunity to update our understanding. We should, even though it takes mental effort.)

MonaLisaOverdrive wrote:

Yes, light is a wave AND a particle, but it's easy to imagine a stream of particles going through two slits at once. It's like pouring dog kibble into two bowls at the same time. Right?

No, the point is even one single photon behaves like a particle and like a wave. Even a single photon goes through both slits.

And it's not that photons are both particles and waves, it's that they behave like both particles and waves. I, at least, used to conjure mental images of tiny billiard balls in my head and then I'd try to superimpose them on mental images of waves, confusing myself in the process. Don't do that.

Imagine that you are singing and dancing on a stage. The blind spectator will say "Ah! Here is a singer"; the deaf spectator will say "Ah! Here is a dancer". Through assiduous work the two improve their respective understanding of the phenomenon, and will end up saying "Ah! Here is a singer-dancer". But while you are indeed singing and dancing (at the same time) you _are_ not a singer-dancer, you _are_ a human, two behaviours whereof were observed concomitantly.

The analogy isn't perfect--particles don't choose what they do--they behave as physics dictates--and they don't sometimes do one thing and other times another. The point is to shake off inadequate visualisations that might have taken up residence in your worldview.

You may now rightfully ask "So what _are_ particles, really? What is the universe actually _of_?" This question hasn't been definitively settled. We have a number of experiments, that are solid, a body of mathematics that we can work with--also solid, though naturally difficult to grok, and a big ass list of possible interpretations of our results.

Finally, I don't know if this will help you understand the double-slit experiment per se, but you should read Configurations and Amplitude, and then you should read about the Elitzur-Vaidman bomb tester, to convince yourself that what the first article says is true. Don't get hung up on how to "interpret" the results just yet. They are what they are. If you start feeling confused, remember that you've been living in a quantum-mechanical universe for the past 14 billion years. No reason to fret now.

and it also just tells you how it works, not why. Without math you can't address the how, you can just answer questions about what happens, for the most part incorrectly 'cause verbal analogies really don't get you anywhere.

You just tell someone - ohh the photons actually go through both holes, and make that pretty interference pattern. Whereas the person can by their direct experiment - addressing your claim as you claimed it - with two holes in piece of cardboard, and lightbulb, see something entirely different (two upside down images of lightbulb filament, ha ha). Hell, without any such experiment a person knows what's going to happen if he/she makes that experiment. So you go on and teach the mystery about abstract photons without explaining how that actually happens (in what conditions, etc), and get entirely tangled up with verbal analogies that just get less and less consistent, as well as get to sound 'counter-intuitive' by simply being mostly false. And that's what happens in popularization books.

edit: ok let me try explaining it in good ol way, referring to the waves which are something one could intuitively understand to some extent. Look at this:http://en.wikipedia.org/wiki/File:Doubl ... ectrum.gifThat's how it may look like if you shine a laser at a piece of foil with 2 tiny holes really nearby, and a screen some distance behind. The laser light is a nice, flat wave, unlike lifhtbulb light, which is a random-ish mess of waves. You can see that the space near screen is waving hard in some points, and stays unchanging in some other points. You get a pattern of intensity on the screen that is behind the slits. That is a proof that light behaves like a wave, the points don't do that.For some odd reason, however, the light is emitted and absorbed always in discrete quantities, called quanta. One quanta of light is one photon. When you do this experiment with single photons fired one by one, what you see is that there is still same wave-produced pattern, after you count enough photons (most photons will be detected at the barrier, btw). That pattern can not happen if the photons are not acting like a big wave that hits the barrier, and then some of it passes through both holes and hits screen. Why it happens - there's really no satisfactory answer to this question. The waving of a single photon somehow actualizes in just one point out of many points where the wave hits stuff, and not anywhere else, and the probability is given by intensity of the waving at that point. Nobody knows how the rest of the wave 'knows' that it already actualized in one point, as to not actualize twice. Each photon behaves like a seriously non-point thing, a wave that is smeared over the space, gets absorbed by surfaces and goes through holes - not just through both holes but also through every point in the each hole - until it interacts with something that the observer is going to look at - then the observer just sees a single dark dot in a photographic emulsion, or a single count with some form of photon counter. But there's never been a dot travelling through space 'through both holes'. There's never been a dot splitting in two to go through both holes, and then merging. There's never been anything merging, either - the wave just got absorbed by the barrier and the screen (by some magic we seen that happen in 1 point per photon).

when a laboratory apparatus was developed that could reliably fire one electron at a time through the double slit,[10] the emergence of an interference pattern suggested that each electron was interfering with itself, and therefore in some sense the electron had to be going through both slits at once[11] — an idea that contradicts our everyday experience of discrete objects. This phenomenon has also been shown to occur with atoms and even some molecules, including buckyballs.[7][12][13]

In order to find the overall probability amplitude for a given process, then, one adds up, or integrates, the amplitude of postulate 3 over the space of all possible histories of the system in between the initial and final states, including histories that are absurd by classical standards. In calculating the amplitude for a single particle to go from one place to another in a given time, it would be correct to include histories in which the particle describes elaborate curlicues, histories in which the particle shoots off into outer space and flies back again, and so forth. The path integral assigns all of these histories amplitudes of equal magnitude but with varying phase, or argument of the complex number. The contributions that are wildly different from the classical history are suppressed only by the interference of similar, canceling histories (see below).

So, according to this a photon leaving an emitter heading for a receiver takes not one, not two, but all possible paths to reach that receiver, with different probabilities for most of these paths, most of them "cancelling out".

Does this answer your question? Or, if not, do you subscribe to a particle-only view? The list of interpretations I linked to in my previous post mentions that there are competing interpretations for the facts we observe.

Or was your question, as Hat Monster inferred, epistemic rather than scientific? Were you asking "how can you prove that it can't be any other way"? If it was, here is not the place for that discussion.

Dmytry wrote:

Without math you can't address the how, you can just answer questions about what happens

Yep. I'm not sure exactly how far strictly verbal explanations can take you before you hit the "Math needed beyond this point" plateau, but that limit is definitely there.

The double-slit experiment is unnecessarily confusing, in my opinion. Historians should learn about it, definitely. Physicists, too, to see where quantum mechanics came from. But for the general public, I think it is completely the wrong story to tell. It leads to all kinds of confusions, as, e.g., in Hat Monster's post---particles and waves and all sorts of irrelevant nonsense. People end up talking about philosophical interpretations when they don't understand what they are talking about. Quantum mechanics is really very simple, simpler than special relativity, for example, but it's being taught the same way it always has, and that is too bad.

Einstein didn't use mathematics to discover relativity. He visualised it. He eventually had to do the maths so that other people could understand it and he could publish his work but he understood it just fine without mathematics.

You could argue that he couldn't be sure he was right without the maths... and that would make sense... but he was right even without using any maths. He visualised how he thouht it would be then used maths to prove that he was correct.

edit: Ok so along the lines of visualisation (not that i'm Einstein or anything) my visualisation of the double slit experiment is thus.Imagine a sea of photons travelling in a wave formation. This is the wavefunction... but each photon is translucent, a ghost photon. It exists... it's real but it's not as real as a photon that has a collapsed wave-function. So long as there is a wavefunction a photon is actually a bunch of ghost photons. Once the wave of ghost photons hits the wall the sum probabilities of their paths place the photon on a spot on the wall. I"m not sure how it works, if 51% of the ghost photons have to hit the wall or 75% or 99% /shrug. Just that you need a majority of the photons to all have reached a spot where they can't move beyond x coordinate... then the ghost photons start dissapearing and at some point enough of them dissapear to collapse the wavefunction and create a real photon.

Ie Reality is fuzzy at the particle level. It only becomes what we think of as reality once you get enough interaction between particles that their wavefunctions collapse and they become non-ghost particles.

Imagine that you are singing and dancing on a stage. The blind spectator will say "Ah! Here is a singer"; the deaf spectator will say "Ah! Here is a dancer". Through assiduous work the two improve their respective understanding of the phenomenon, and will end up saying "Ah! Here is a singer-dancer". But while you are indeed singing and dancing (at the same time) you _are_ not a singer-dancer, you _are_ a human, two behaviours whereof were observed concomitantly

Now I understand. Thanks Apteris!

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You may now rightfully ask "So what _are_ particles, really? What is the universe actually _of_?" This question hasn't been definitively settled. We have a number of experiments, that are solid, a body of mathematics that we can work with--also solid, though naturally difficult to grok, and a big ass list of possible interpretations of our results

Will it ever be settled, do you think? Especially if...extending your analogy...a significant portion of information is obscured from us?

I must admit...the discovery of such an interesting field makes me regret, even more, my complete hatred of mathematics during my middle, high-school and college years. Pretty sure it would help me understand this on a deeper level than I can now...and I'm fairly sure I'm too old to start on it now.

From a classical view, one would expect a single particle double slit experiment to yield two bands. The exact position of the particle is random, but if you do the experiment enough times, two bands are expected to appear. You can verify this with a paintball gun, two slits of appropriate size and a blank wall (or a similar set up). Just stand in one place and shoot forwards.

However, with a quantum particle, like a photon or electron, the distribution is identical to an interference pattern. The only way a single particle can build up an interference pattern is if can interfere with itself ie it's a wave (or has a wave nature).Why and how this happens is a bit of a mystery, but it definitely does happen.

Regarding wave-particle duality, there's no satisfactory explanation of what exactly it means. Suffice to say, quantum properties depend on the measurement technique. As others have said, measuring the path of the photon destroys the interference pattern.Stephen Hawking's idea of model dependent realism may be useful.

Einstein didn't use mathematics to discover relativity. He visualised it. He eventually had to do the maths so that other people could understand it and he could publish his work but he understood it just fine without mathematics.

You could argue that he couldn't be sure he was right without the maths... and that would make sense... but he was right even without using any maths. He visualised how he thouht it would be then used maths to prove that he was correct.

Visualisation is rooted in mathematics, in particular, geometry... if i visualize linkage between crankshaft and piston, rotated, I can't help it but have some analytic geometry happen somewhere in the head so that the mechanics works right. Then i can sit down and write the equations for the piston position vs time if i want.

One thing that really, really, really does not work for physics though (apart from as applied to manipulating equations) is verbal string processing.

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edit: Ok so along the lines of visualisation (not that i'm Einstein or anything) my visualisation of the double slit experiment is thus.Imagine a sea of photons travelling in a wave formation

I imagine single photon being a big, faint wave - a lot of coherent photons, as from laser, being a big, brighter wave. Wave hits the barrier with holes, gets absorbed, it hits the screen, gets absorbed... imagine all wave's impact upon detector (but NOT wave in the empty space), for inexplicable reason, always getting concentrated in N observable points instead of smeared over - the sparks that are of same brightness for same wavelength. A single bright spark for single photon. You have stronger wave, you see sparks more often, the spark's probability is dependent to wave's intensity at that point. Imagining a single pre-existing real photon somewhere specific on the wave, going till it hits the screen, that's a hidden-variable theory, and very surely incorrect (see Bell's experiment). There isn't really a wave particle duality per se. There is a wave - we know that, it works like a wave - which does very strange stuff when it hits things - it hits things in discrete punches, as far as we see*. The usual 'particles' like bullets also hit things in sort-of discrete punches, hence we speak of particles as in wave particle duality and such. But the photons in question do not behave anything like bullets apart from delivering discrete punches.*which simply isn't a full picture, under MWI.

Imagining a wave is somewhat mathematical already, if you can imagine it well enough as to 'simulate' things this way - imagining two waves and adding them up and seeing the interference pattern.

Will it ever be settled, do you think? Especially if...extending your analogy...a significant portion of information is obscured from us?

I think so. But even if the question isn't settled, we'll still have repeatable experiments and the math needed to make sense of them. That's comforting.

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I must admit...the discovery of such an interesting field makes me regret, even more, my complete hatred of mathematics during my middle, high-school and college years. Pretty sure it would help me understand this on a deeper level than I can now...and I'm fairly sure I'm too old to start on it now.

Everybody needs a hobby, right? Spend a few hours with a good math book, see how you feel about it. Motivation trumps age.

1. Photons do not experience time, so if you're a photon you have no concept of "before" or "after". You can quite easily interfere with photons which came before you or will come after you.

2. Virtual photons appear in the mathematics and can interfere with "real" photons. This is one of the more intuitive explanations, which says quite a bit about quantum mechanics.

3. 1 + 2 above. Virtual photons can be all the past and future photons "existing" at present, as time is meaningless to a photon.

4. As a wavefunction, the photon has a probability non-zero in both slits and can travel through them both at the same time.

Generally among optical engineers who are not physicists (and thus do not intuitively grasp QM), the "explanation" is that the EM field goes through both slits, but only has a total energy of one photon. In this interpretation the underlying field is the real thing, and the photon is just the smallest quanta of energy the field can increment/decrement in.